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I'm o positive and from what I understand that's the rare one. I haven't a clue as to why thre are different types. I remember from biology class though that if one parent is type A and the other is type B the child is born AB. So all of your siblings that have the same mother and father are the same type as you. I think this is also why when you need an organ transplant, the first choice they look to for a match are your siblings.

Oh and it's either O+ or O- or both that can be used as a blood transfusion with all types of blood, no matter what the person has. That's one of the reasons when they ask you to give blood that they are in desperate need of O.

It is interesting. It is odd that you asked this question because I was just researching the subject. I am pregnant and am having some issues due to blood type so that is the only reason why I knew the answer.